Current methanol-to-olefin processes typically are designed in a manner that produces ethylene and/or propylene. The by-products from the methanol-to-olefin processes, however, usually include C4, C5, and C6+ streams that can be rich in olefins. A typical composition may include a weight ratio of C4/C5/C6 olefins of about 5:2:1. These C4+ streams often include at least 80 weight % of olefins. These by-products are typically disposed of as fuels of various types, or converted to fuel components, such as octane enhancers, normally after being separated into two or more fractions. It would be advantageous, however, to convert these streams into commercially valuable olefins, such as propylene and/or ethylene.
Even though it is theoretically possible, these streams typically are not suitable for use as an olefin cracker feedstock, due at least in part to the high content of olefins. It is also possible that these streams may be used for producing light olefins via metathesis processes, but some isomers in each group of the C4, C5, and C6 olefins are undesirable. Furthermore, the oxygenates and dienes/acetylenes contained in the streams, which are inherent of MTO processes, should be removed prior to the foregoing processes, which results in increased cost, increased waste generation, or a combination thereof.
Methods are known for producing commercially important olefins, such as ethylene and propylene. Such methods include steam cracking, propane dehydrogenation, and various refinery catalytic cracking operations. Each of these procedures has one or more disadvantages. For example, propylene yields from steam cracking typically are not very high, and usually are not substantially improved by recycling. Also, purification of non-propylene products may be required, which can be costly, and such products usually have only fuel value. Propane dehydrogenation processes usually are characterized by rapid catalyst coking, which can require frequent, costly regenerations. Also, reasonable conversions typically require sub-atmospheric pressures, and propane can be difficult to separate from propylene. Moreover, propylene supplies from catalytic conversions are uncertain, and transportation and/or purification can present problems.
Therefore, methods are desired that convert the by-products, including pre-fractionated by-products, of chemical processes, such as methanol-to-olefin processes, to ethylene and/or propylene in an efficient, cost-effective, and/or facile manner.